The invention relates to a method of preparing powdered elastomer compositions which is an extension of and an improvement over the methods described in commonly assigned U.S. Pat. Nos. 3,830,762; 3,442,832; 3,480,572; 3,645,940; 3,673,136; and 3,714,087 which are incorporated herein by reference.
The invention has for its objects and advantages all the objects and advantages disclosed in U.S. Pat. No. 3,830,762, supra. It is a further object of the invention to provide a method of preparing powdered elastomer compositions which contain sufficient amounts of encasement compounds to completely encase the elastomer particles leaving them nontacky and, therefore, nonagglomerating and free flowing (T. P. Abbott et al., "Starch-Encased Rubbers: Injection Molding of Conventional Powdered Rubber Formulations," J. Elastomers Plast., April 1975, incorporated herein by reference).
In accordance with the above objects, I have discovered in a process for preparing powdered elastomer compositions having the following steps:
A. coprecipitating from 3 to 19 parts of a first encasement compound selected from the group consisting of starch xanthate and zinc starch xanthate having xanthate degrees of substitution of from 0.2 to 3.0, cationic starch and their cereal flour analogs with 100 parts of solids contained in a latex selected from the group consisting of natural rubber latex, butadiene-acrylonitrile rubber latex, styrene-butadiene rubber latex, chloroprene rubber latex, polybutadiene rubber latex, oil-extended styrene-butadiene rubber latex, and phenolformaldehyde-extended nitrile rubber latex to form a coprecipitate having a particle diameter of about 1 mm.; PA1 B. filtering and water washing the coprecipitate resulting from (a); PA1 C. washing the coprecipitate resulting from (b) with a water-miscible solvent; PA1 D. filtering the washed coprecipitate resulting from step (c); and PA1 E. repeating steps (c) and (d) until the filtrate resulting from step (d) contains from 0 to 10 percent water; PA1 1. Elastomer (i.e., rubber) latex: natural, styrene-butadiene (SBR), acrylonitrile-butadiene (NBR), chloroprene (CR), polybutadiene, and oil-extended SBR [see Kirk and Othmer Encyclopedia of Chemical Technology, Vol. 7, pp. 676-716, and Vol. 17, pp. 660-684, The Interscience Encyclopedia, Inc., New York (1965) for a discussion of natural and synthetic elastomers]; PA1 2. First encasement compounds: starch xanthate, zinc starch xanthate, cationic starches such as aminoalkyl and quaternary ammonium alkyl ether derivatives of starch [see Starch: Chemistry and Technology, ed. Whistler and Paschall, Vol. II, Chapter 16, Academic Press, New York (1967)], cationic starch graft copolymers of the type described in U.S. Pat. No. 3,669,915, and cereal flour analogs of the above; PA1 3. Precipitation reagents: ZnSO.sub.4, ZnCl.sub.2, or the equivalent mineral acid.
An improvement comprising, as an additional step prior to step (c), supra, resuspending the coprecipitate resulting from step (b), supra, in water and precipitating, in the presence of the resuspended coprecipitate, from 1 to 17 parts per 100 parts of latex solids (phr) of a second encasement compound selected from the group consisting of starch xanthate and zinc starch xanthate having xanthate degrees of substitution of from 0.2 to 3.0, cationic starch, their cereal flour analogs, and sodium stearate, the total amount of first and second encasement compounds being from 4 to 20 phr. Steps (c) through (e), supra, constitute "dehydration of the doubly encased elastomer coprecipitates."
An added advantage of the invention is the provision of nontacky powdered elastomer compositions encased in starch-containing compounds in amounts sufficiently low so that the starch contained therein does not act as a filler. In quantities of less than 8 phr, starch has little or no effect on the properties of filled vulcanized rubber, and only small effects on nonfilled rubber (e.g., gum rubber). When powdered elastomer containing over 10 phr starch are used to replace slab rubber in injection molding compositions, the amount of filler in the formulation must be adjusted to compensate for the amount of starch. The amounts of other ingredients also may have to be adjusted to obtain a rubber product having the desired properties.